The present invention generally relates to semiconductor devices and more particularly to a planar semiconductor optical device used for modulating an optical beam in response to an electric signal supplied thereto.
In the field of optical telecommunications and optical information processing, optical modulating elements for modulating an incident optical beam in response to an electric signal is needed. For example, in switching of optical signals between two or more optical paths, it is practiced to provide an optical coupler that couples two or more optical waveguides and to modulate the refractive index of the optical waveguides such that the optical signal is transferred from one optical waveguide to the other optical waveguide, or vice versa.
On the other hand, in order to handle the interconnection of large number of optical paths simultaneously, it is advantageous to provide optical modulation elements in row and column formation to form a two-dimensional array. There, each modulation element is connected to a corresponding optical waveguide, optical fiber or a lens, and each modulation element controls the passage of the optical signal therethrough.
In order to realize such an optical modulation element, intensive efforts are being made and various devices are proposed so far. Particularly, the so-called quantum optical devices that create quantum levels or exciton levels therein for interaction with the supplied optical beam are considered most promising and various studies have been made. In the quantum optical devices, the carriers such as electrons and holes that interact with the incident optical beam are confined at least in one of the three-dimensions with a size comparable to the de-Broglie wavelength of the carriers. As a result of the carrier confinement, there appears discrete quantum levels as is well known in the art. Further, such a structure for confining the carriers generally has a size comparable to or smaller than the Bohr radius of the electron-hole system and creates clear and discrete energy levels for excitons even in the room temperature environment. In the bulk crystals that lack the quantum confinement of carriers, the energy spectra of excitons can be observable only at the extremely low temperatures.
The quantum devices characterized by the exciton levels are susceptible to irradiation of incident optical beam in that the incident optical beam, when having a wavelength matching the energy level of the excitons in the device, cause an excitation of electrons and holes to form excitons, and the excitons thus formed cause a decomposition or ionization to form electrons and holes. In other words, the electrons and holes are released upon absorption of the incident optical beam. The electrons and holes thus released occupy respective quantum levels and the quantum levels are saturated with the electrons and holes with the progress of ionization of the excitons. Thereby, the absorption of the optical beam by the excitons no longer occurs. In other words, the device shows a strong non-linearity in the optical absorption and hence in the refractive index.
FIG. 1 shows an example of the optical modulating element that is designed for use in the two-dimensional optical modulation array.
Referring to FIG. 1, the device is constructed on a p.sup.+ -type GaAs substrate 11 covered by a buffer layer 12 of p-type GaAs with a thickness of about 3000 .ANG.. On the buffer layer 12, an active layer 13 having a MQW structure is provided with a thickness of about 2500 .ANG., wherein the MQW layer 13 is formed by a repetitive deposition of a layer unit that consists of an undoped GaAs barrier layer 13a having a thickness of 150 .ANG. and an undoped InGaAs quantum well layer 13b having a composition of In.sub.0.13 Ga.sub.0.87 As and a thickness of 100 .ANG.. The layer unit is repeated for ten times to form the MQW layer 13, wherein, in the layer 13, the InGaAS quantum well layer 13b characterized by a small bandgap Eg.sub.1, is sandwiched by a pair of GaAs barrier layers 13a having a larger bandgap Eg.sub.2. Thereby, discrete quantum levels are formed in the quantum well layer 13b with a mutual energy separation determined by the thickness of the quantum well layer 13b. It should be noted that the thickness of the active layer 13b is set such that the quantum levels formed therein correspond to the optical energy of the incident optical beam that is supplied to the device for optical modulation. On the MQW layer 13, a contact layer 14 of n.sup.+ -type GaAs is deposited with a thickness of about 3000 .ANG.. Further, a p-type ohmic electrode 15 of a Au/Zn alloy is provided on the lower major surface of the substrate 11, and an n-type ohmic electrode 16 of a Pd/Ge alloy is provided on the upper major surface of the contact layer 14.
There, the electrodes 15 and 16 are provided with respective windows 15a and 16a for the optical beam, wherein the window 15a exposes the lower major surface of the substrate 11 while the window 16a exposes the upper major surface of the contact layer 14. Thereby, the incident optical beam is supplied to the window 16a as indicated in FIG. 1 and exits from the device via the window 15a, after experiencing the optical modulation at the MQW layer 13.
In operation, a reverse bias voltage is applied across the electrodes 15 and 16 from a d.c. voltage source 17 such that a potential gradient or electric field is induced within the MQW layer 13. Thereby, the absorption wavelength of the MQW layer 13 shifts by a mechanism such as the Franz-Keldysh effect or quantum confinement Stark effect, and the absorption of the incident optical beam, initially tuned to the absorption wavelength of the MQW layer 13, changes. In other words, a modification of the optical beam is achieved in response to an electric signal supplied to the device.
By forming the device of FIG. 1 on a common substrate in a row and column formation, one can realize a desired two-dimensional array of the optical modulating device. In the device of FIG. 1, it will be noted that it is also possible to obtain the desired optical modulation by injecting carriers to the MQW layer 13 by applying a forward bias voltage across the electrodes 15 and 16 Thereby, the refractive index and hence the absorption of the MQW layer changes by the plasma effect.
In the device of FIG. 1, one may provide a Bragg reflector in correspondence to the layer 12 for reflecting back the incident optical beam. Typically, the Bragg reflector used for the layer 12 consists of an alternate stacking of first and second layers forming a unit of repetition, wherein the first and second layers have a total thickness, in each unit of repetition, equal to one-quarter the wavelength of the incident optical beam in each layer. In other words, the Bragg reflector layer is tuned to the incident optical beam and selectively reflects the optical beam having a wavelength to which the Bragg reflector is tuned. Thus, when the MQW layer 13 absorbs the incident optical beam, the reflection of the incident optical beam does not occur, while when the MQW layer 13 is biased, the incident optical, beam reaches the Bragg reflector at the layer 12 and is reflected back.
In the construction of the device of FIG. 1, it will be noted that the MQW layer 13 is exposed laterally such that there is no optical confinement for those optical beams that have a component propagating laterally within the MQW layer 13. In other words, the structure of FIG. 1 lacks the construction for preventing the incident optical beam from spreading laterally. Associated with this problem, the conventional optical modulating devices have suffered from the problem of large optical loss, particularly to those optical beam components that have entered the device obliquely. Thus, the problem of optical loss is particularly serious in the optical modulation devices that use a Bragg reflector for reflecting the optical beam back and forth a number of times. It will be noted that the optical beam tends to spread laterally away from the region that is aligned with respect to the windows 15a and 16a, when the reflection is repeated a number of times. Thereby, the optical loss increases and the efficiency of modulation of the optical beam is inevitably deteriorated.
It should be noted that the structure of FIG. 1 has another drawback in that the active part of the device tends to be affected by the environmental condition such as moisture because of the side wall of the active layer exposed to the environment. Further, such a structure is mechanically fragile.
Although it is known in the art to cover the optically active parts by the deposition of silicon oxide film to achieve an optical confinement as well as protection, the thickness of such a silicon oxide layer generally cannot exceed more than 1000 .ANG. particularly when deposited on a compound semiconductor material as shown in FIG. 1. In order to achieve an effective optical confinement, one needs a thickness of at least 1 .mu.m, preferably 2-3 .mu.m for the silicon oxide layer. When silicon oxide is deposited on the side wall of the optical semiconductor device as shown in FIG. 1 with such a large thickness, various problems such as cracking or detachment occurs in relation to the mismatching of various physical parameters such as thermal expansion coefficient. Associated therewith, various adverse effects such as build-up of strain occur also on the active part of the device and the optical property of the device is inevitably deteriorated.